1. Field of the Invention
The invention relates to a super-high pressure discharge lamp of the short arc type in which the mercury vapor pressure during operation is at least 15 MPa. The invention relates especially to a super-high pressure discharge lamp of the short arc type which is used as the backlight of a liquid crystal display device, a projection device such as a DLP (digital light processor), or the like, in which a DMD (digital mirror device) is used.
2. Description of Related Art
In a projector device of the projection type, there is a demand for illumination of the images uniformly onto a rectangular screen with sufficient color reproduction. The light source is thus a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently smaller and smaller metal halide lamps, and more and more often spot light sources, have been produced and lamps with extremely small distances between the electrodes have been used in practice.
Against this background, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, with 15 MPa, have recently been proposed. Here, the increased mercury vapor pressure suppresses broadening of the arc (the arc is compressed) and a major increase of the light intensity is desired.
One such super-high pressure discharge lamp is disclosed, for example, in Japanese patent disclosure document HEI 2-148561 (corresponding to U.S. Pat. No. 5,109,181) and Japanese patent disclosure document HEI 6-52830 (corresponding to U.S. Pat. No. 5,497,049).
In such a super-high pressure discharge lamp, the pressure in the arc tube during operation is extremely high. In the side tube portions which extend from opposite sides of the arc tube portion, it is therefore necessary to arrange the silica glass comprising these side tube portions, the electrodes and the metal foils for supply in a sufficient amount, and moreover, almost directly tightly adjoining one another.
Especially since electrodes are generally cylindrical and metal foils are plate-shaped, when the two are joined to one another, in the areas bordering the silica glass, extremely small gaps always form via which high gas pressure in the emission space is applied into the vicinities of the electrode rods; this can lead to the formation and growth of cracks.
Therefore, to prevent crack formation, it becomes a more and more important task how to make this gap smaller. The attempt is made to make smaller the extremely small gap which forms in the vicinity of the electrode rods by reducing the cross sectional area of the electrode rods.
One such gap which is formed in the vicinity of the electrode rods is described, for example, in Japanese patent disclosure document HEI 3-201357.
On the other hand, a super-high pressure discharge lamp of the short arc type which is used in a projector device is subject to extremely severe thermal conditions, the internal air pressure during operation is at least 15 MPa and the value of the wall load is at least 0.8 W/mm2, even if the inner volume of the arc tube is extremely small, e.g., is roughly 80 mm3. Therefore, during operation of the discharge lamp, a heat dissipation measure for preventing a temperature increase of the discharge vessel must be taken to an adequate degree to prevent devitrification.
As this heat dissipation measure, it can be imagined that cooling air or the like can be blown in from outside the discharge vessel. However, as another measure, heat dissipation by heat transfer of the electrodes (electrode rods) is an important element.
If only heat conduction and radiation within the discharge space is mentioned, the heat dissipation effect is better, the thicker the electrode rods (the larger the cross sectional area).
A summary of the aforementioned is described below.
In a super-high pressure mercury lamp of the short arc type for a projector, with extremely severe thermal conditions where the gas air pressure during operation within the discharge vessel is extremely high (for example, at least 15 MPa), the internal volume of the arc tube is at most 80 mm3, and that the wall load is at least 0.8 W/mm2, there are, first of all cases, in which, due to the high filler gas pressure during operation in the side tube portions, cracks form and grow which never form in a normal discharge lamp (with a gas pressure during operation of roughly a few atm to a few dozen atm). It is therefore desirable to reduce the size of the extremely small space which causes the formation of cracks by reducing the diameter of the electrode rod.
Secondly, the high temperature within the discharge space must be quickly subjected to heat dissipation since the thermal conditions during operation are extremely strict. Therefore, it is important to use the action of heat transfer by the electrode rods. As a specific arrangement it is desirable to make the electrode rods thick.
One means for achieving these objects is disclosed, for example, in Japanese patent disclosure document HEI 10-289690. In this patent disclosure document it is disclosed that the diameter of the electrode rod of the area in which it is welded to the glass, compared to the area in which the discharge arc is fixed, is smaller and that the diameter of the electrode rod proceeding from the area in which the discharge arc is held is incrementally or continuously reduced in size in the direction to the weld with the glass.
This arrangement is intended to achieve the two above described objects both qualitatively. In the discharge lamp disclosed in this patent disclosure document, the internal pressure of at least 0.1 MPa is a very low (1st paragraph in the description in the application documents). Therefore, for a discharge lamp with a high internal pressure, for example, of at least 15 MPa, i.e., with an internal pressure which is two orders of magnitude greater, as for the discharge lamp of the short arc type in accordance with the invention, the objects could not always be completely achieved.
The object of the invention is to devise an arrangement with relatively high pressure tightness in a super-high pressure mercury lamp which is operated with an extremely high mercury vapor pressure.
The object is achieved, in accordance with a preferred embodiment of the invention, in a super-high pressure mercury lamp of the short arc type which comprises the following:
an arc tube portion in which there is a pair of opposed electrodes, with tungsten as the main component, and which is filled with at least 0.15 mg/mm3 mercury and
side tube portions which extend from opposite sides of the arc tube portion and in which the electrodes are partially hermetically sealed, and in which the electrodes and metal foils are welded to one another,
in that the above described electrodes and the above described metal foils are each electrically connected to one another by means of a metallic component as an individual body with a smaller cross sectional area than the cross sectional area of the above described electrodes.
The object is furthermore achieved in a super-high pressure mercury lamp of the short arc type in that the above described metallic component has a diameter from 0.1 mm to 0.5 mm.
The object is also achieved in a super-high pressure mercury lamp of the short arc type in that an extremely small space is formed in the above described side tube portions between the side and the end face of the above described respective electrode and the silica glass comprising these side tube portions.
Still further, the object is achieved in a super-high pressure mercury lamp of the short arc type which comprises
an arc tube portion in which there is a pair of opposed electrodes, with tungsten as the main component, and which is filled at least 0.15 mg/mm3 mercury and
side tube portions which extend to opposite sides of the arc tube portion and in which there are metal foils,
in that in the area opposite the above described respective side tube portion, the respective above described electrode with the material which comprises this side tube portion forms an extremely small gap and that the above described respective electrode is made of a part with a larger diameter which is opposite this material component, and of a part with a smaller diameter which is welded to the above described metal foil.
The object is furthermore achieved in a high pressure mercury lamp of the short arc type in that the part with a larger diameter of the electrode has a diameter from 0.6 mm to 1.5 mm and that the part with a smaller diameter of the electrode has a diameter from 0.1 mm to 0.5 mm.
The invention is explained in greater detail below using several embodiments shown in the drawings.